Why Are There Sight Glasses in Refrigerators?

No it's not for filling... it's to verify flow.

... and to see if you have air in the refigerant. (read bubbles)

Sight glasses used to be used for charging refrigerant but with the advent of the new non ozone depleting CFC the sight glass is only to check the flow of liquid refrigerant from the condensor. In my experience I have found very few sight glasses in domestic refrigerators. I assume we are talking about a chiller or medium sized cold room. If you are charging R12 or R22 you can use the sight glass as an indication of correct charge by observing the flow of liquid untill you have no bubbles. The correct charge should also have a slight sweat on the suction line. The newer refrigerants however are charged purely by pressure [ eg R134 R409A ]. You must observe the head pressure and suction pressures and ambient temperature of the day and work the pressures out accordingly. You must use the Press./ Temp tables for the gas and arrive at suitable head pressure which will indicate correct charge.